1. Field of the Invention
The invention generally relates to semiconductor devices such as semiconductor lasers, light emitting diodes, and heterojunction bipolar transistors. More specifically, the invention relates to efficient injection of electrons or holes from a wider bandgap semiconductor material to a narrower bandgap semiconductor material.
2. Related Technology
Vertical cavity surface emitting lasers (VCSELs), surface emitting lasers (SELs), light Emitting Diodes (LEDs), and heterojunction bipolar transistors (HBTs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications.
VCSELs, SELs, and LEDs are generally formed as a semiconductor diode. A diode is formed from a junction between a p-type material and an n-type material. In VCSELs, the p-type semiconductor material is most often aluminum gallium arsenide (AlGaAs) doped with a material such as carbon that introduces free holes or positive charge carriers, while the n-type semiconductor material is typically AlGaAs doped with a material, such as silicon, that introduces free electrons, or negative charge carriers.
The PN junction forms an active region. The active region typically includes a number of quantum wells. Free carriers in the form of holes and electrons are injected into the quantum wells when the PN junction is forward biased by an electrical current. At a sufficiently high bias current, the injected minority carriers form a population inversion in the quantum wells that produces optical gain, which is used inside a resonant cavity to cause lasing. The resonant cavity is formed by properly spacing mirrors on either side of the active region.
Free carriers that escape the quantum wells into the surrounding semiconductor material and recombine there do not contribute to the optical gain. These events are parasitic currents that generate heat and reduce the efficiency of the light emitting device. This “carrier leakage” is one of the causes of the rollover of the light vs. current curve. Current can only be increased so much and then light output reaches a maximum and drops off. Generally, higher temperatures result in lower maximum light output partially because the thermal energy of the carriers, electrons, and holes is increased allowing a larger fraction to contribute to carrier leakage. Electrical confinement in the active region can be particularly problematic in VCSEL devices, which tend to require high current densities for operation and is made worse in the highest frequency VCSELs where the highest current densities are used.
To improve current confinement, most semiconductor lasers have confining layers next to the active region. The confining layers have a bandgap that is substantially wider than the bandgap of the quantum wells and quantum well barriers. For carriers to escape from the active region, the carriers need higher energy to pass through the confining layer. The higher energy requirements in the confining layer make it more likely that carriers will remain in the active region and contribute to stimulated emission at the desired wavelength.
One potential concern with confining electrons in the active region is the effect that the confining layer has on the injection of carriers into the active region. In some cases, measures taken to confine carriers in the active region can also decrease the efficiency of injecting the carriers into the active region.